The digital transmission system Digital Radio Mondiale (DRM System) was developed for frequency bands below 30 MHz by a worldwide consortium, and was standardized in the year 2001 (ETSI TS 101980). The DRM system was designed by transmission technology and from transmission parameters especially for long wave, middle wave and short wave. Meanwhile, attempts are being made for a broadening of the system to include the VHF band. However, since propagation properties in this broadened frequency band differ substantially from those of the usual frequency bands, the system has to be broadened for effective transmission.
In the DRM transmission system, an important feature is alternative frequency switching. By alternative frequency switching is understood the switching of a transmitted signal at a first carrier frequency to a send signal at a second carrier frequency. Such a switching is carried out, for example, when reception quality is deteriorating on the first carrier frequency or is low, and a program of the same information content is being transmitted on an additional carrier frequency. Such a procedure is also known in connection with the usual analog broadcasting as FM-RDS.
Since a single receiver system is usually involved in the DRM system which is only able to receive and decode one transmitted signal at one carrier frequency at one point in time, one problem is switching between carrier frequencies in such a way that, for instance, in radio reception, no audible interference of the output signal occurs, or rather, that the transmitted data stream is received essentially completely.
As transmission method, the DRM system utilizes OFDM (orthogonal frequency division multiplexing), which is known from the related art. The transmission of the data takes place with the aid of successively sent OFDM symbols. In adapting the DRM system to broadened carrier frequencies, it is meaningful to select the OFDM symbol duration different, based on other channel properties. Therefore, in order to broaden the DRM system to carrier frequencies in a wider transmission band, new OFDM parameters thus have to be established. In the case of higher carrier frequencies, such as in the VHF band, the symbol duration, for example, should be selected to be substantially shorter. As a result, a larger number of OFDM symbols is sent in a certain time period.
In order to be able to decode the OFDM symbols, a renewed channel estimate is necessary when switching to such a further carrier frequency, since the channel estimate cannot be continued on the carrier frequency of the transmitted signal, and there is not yet present a current channel estimate for the send signal on the further carrier frequency to which switching is taking place.